An SOI (Silicon On Insulator) substrate, a silicon substrate having a monocrystalline silicon layer formed on the surface of an insulating layer, has been known in the art. Reduced parasitic capacitance and increased insulation resistance can be achieved by forming a device such as a transistor on an SOI substrate. In other words, higher device integration and higher device performance can be obtained. The insulating layer is formed from, e.g., a silicon oxide film (SiO2).
For a higher operating speed and further reduced parasitic capacitance of a device, it is desirable that the SOI substrate have a thinner monocrystalline silicon layer. Therefore, in a known method for fabricating an SOI substrate, a silicon substrate is first bonded to another substrate such as a glass substrate and a part of the silicon substrate is then separated and removed (for example, see Non-patent reference 1).
This method for fabricating an SOI substrate by bonding will be described with reference to FIGS. 10 through 13. Although the thickness of an SOI layer can be reduced by various methods such as those using mechanical polishing, chemical polishing, and porous silicon, a method using hydrogen implantation will be described herein. First, as shown in FIG. 10, a silicon oxide (SiO2) layer 202, an insulating layer, is formed by oxidizing the surface of a silicon substrate 201, a first substrate. Thereafter, as shown in FIG. 11, hydrogen is implanted as a separating material into the silicon substrate 201 through the silicon oxide (SiO2) layer 202 by ion implantation. A hydrogen implantation layer 204 is thus formed as a separation layer at a prescribed depth in the silicon substrate 201. The substrate surface is then cleaned by RCA cleaning or the like. As shown in FIG. 12, a second substrate, for example, a silicon substrate 203, is then bonded to the surface of the silicon oxide layer 202. The following heat treatment produces microcracks at the hydrogen ion implantation depth. As shown in FIG. 13, a part of the silicon substrate 201 is then separated along the hydrogen implantation layer 204. The thickness of the silicon substrate 201 is thus reduced, whereby a silicon layer 201 is formed. It should be noted that, after the separation, the substrate is reduced to a desired thickness by various methods such as polishing and etching, as required, and also, a process of repairing crystal defects resulting from hydrogen implantation, a process of smoothing the silicon surface, and the like are conducted by heat treatment or the like, as required.
An SOI substrate having the SiO2 layer (insulating layer) 202 on the surface of the silicon substrate (second substrate) 203 and having the thin silicon layer 201 on the surface of the SiO2 layer 202 is fabricated in this way.    Non-patent reference 1: Michel Bruel, “Smart-Cut: A New Silicon On Insulator Material Technology Based on Hydrogen Implantation and Wafer Bonding,” Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 1636-1641.